This invention relates to methods of patterning electronic and photonic materials, in particular those deposited using solution-deposition techniques, such as semiconducting polymers, and to structures and devices fabricated using these methods.
In the past decade solution-processed semi conducting polymers have become an attractive class of materials for plastic electronics because they are easily processible for low-cost, large-area devices. There has been tremendous progress on materials design, device architecture and fabrication, and understanding of the charge transport mechanism in these materials. Particularly, the development of semi crystalline semi conducting polymers such as poly(3-hexylthiophene) (P3HT), poly[5,5′-bis(3-alkyl-2-thienyI)-2,2′-bithiophene)] (PQT), and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-blthiophene) (PBTTT) for solution-processed thin film transistors (TFTs) has enabled achieving mobilities of 0.01-1 cm2 Ns approaching those of amorphous silicon or small molecule transistors. The high mobilities in these semi crystalline semi conducting polymers are primarily attributed to the formation of preferentially in-plane oriented n-n stacking that leads to efficient in-plane charge transport. The highly crystalline structures and high bulk conductivity in these polymers, however, leads to non-negligible leakage currents through the bulk of the film which result in low ONOFF current ratios of the transistors if the semi conducting film is un-patterned. It also causes undesirable crosstalk in realization of integrated circuits. It is thus necessary to develop techniques for patterning semi conducting polymer thin films to prevent the formation of conductive pathways between individual devices.
Various techniques have been developed to pattern polymers such as direct writing by scanning probe microscopy, ink-jet printing, nanoimprinting, microcontact printing, dry-etching process by laser ablation, and photolithography. These techniques have great advantages in achieving relatively high resolution (microns to 35 nanometers) but it can be challenging to avoid compromises and trade-offs between the processing requirements to achieve best patterning and the processing conditions to achieve optimum device performance. Problems often include polymer degradation during patterning induced by, for example, photoirradiation, usage of chemicals or solvents incompatible with polymers, or the need to perform the patterning in ambient environment. On the other hand, a few approaches have been developed that avoid materials degradation and enable patterning of “sensitive” polymer semi conductors which are prone to oxidation in air and/or whose performance is very sensitive to polymer microstructure, substrate roughness, film deposition conditions, solvent exposure etc. These include patterning involving selective dewetting, phase separation, transfer printing, and using innovative materials which are compatible with polymer semiconductors in conventional lithographic patterning. In particular, transfer printing has been used to pattern small molecule and polymer semiconductors for device applications. This approach mainly utilizes a hard master or a poly(dimethylsiloxane) (PDMS) mold to pick up a semi conducting polymer film from a first substrate and transfer it onto another substrate, either as a continuous film or involving patterning. In the latter approach patterning is achieved by picking up the polymer film selectively from the first substrate by using a suitably patterned mold with its surface modified by glycerol. The benefits of this transfer printing approach are high resolution and compatibility with multilayer patterning. However, the specific surface properties of the transfer mold and substrates, for example, the glycerol modification required for picking up and transferring the polymer film, might influence the surface/interface quality of the patterned semi conducting polymer and impact device performance adversely if the polymer semiconductor is “sensitive”.
Hence there is a need for new, simple patterning techniques that enable high resolution patterning of semi conducting polymers that do no impact adversely on the final device. It has now been surprisingly found that patterning may be based on selective physical delamination instead of chemical patterning or etching processes. The delamination processes can be easily integrated into device fabrication under inert atmosphere and can avoid degradation of semi conducting polymers induced by oxygen exposure and chemicals. The spatial resolution of the technique may be defined by a lithographic step performed prior to polymer deposition.